NoD
I agree. On a cost return basis it makes no sense at all. I have no idea how many flying machines there are in the UK without transponders - lets say it is 25,000 - at £4,000 a shot to fit a mode S that is a £100M business. Based on the stats that might save a dozen lives or so over its life time - assuming for that matter every mid air would be avoided if everyone had mode s - which it would not.
Of course as a society we choose not to measure safety always in terms of cost but the "return" for pilots would be abysmal (compared with say the cost of fitting seat belts to cars).
The reality is harsh and it is not much help if you are one of the very unfortunate few.
However expenditure of this sort is not always rational. How many pilots spend £1,000 on a life raft and how many ditchings occur each year? How many yachties carry a liferaft and how many end up needing it? How many yachts are now fitting transponders and how many life threatening collisions occur at sea? (There was one last year in UK waters that I know about).